1. Field of the Invention
This invention relates to motor controllers, and, in particular, to four-quadrant control of series, compound, and shunt wound direct current (DC) motors connected to a DC power source. The invention also relates to four-quadrant control of DC motors connected to a receptive DC power supply.
2. Background Information
Since the early 1900""s, current in direct current (DC) motors was controlled by switching resistors in series with the motor""s armature and field in order to obtain variable speed or torque as required by a particular application. This method was wasteful of energy, and did not provide very good speed or torque regulation.
During the early 1960""s, solid state controllers using SCRs or thyristors were introduced which improved efficiency as well as speed and torque control accuracy. The initial controllers were first used with DC shunt motors and AC power sources, wherein SCRs were employed in AC/DC controlled rectifier configurations with appropriate current and voltage feedback devices.
During the late 1960""s, impulse-commutated SCR converters became available which controlled DC motor current and/or voltage when connected to a DC power source. These converters were used mainly for crane and electric vehicle applications powered from DC sources, such as rectified AC sources, or, in the case of some electric vehicles, on-board batteries. In the case of off-board DC power sources, equipment incorporating DC motors was connected by a shoe sliding on a powered collector, rail or overhead wire, or by trailing/festooned cables.
Since it was difficult to obtain good control with such SCR technology when used together with DC series motors during braking operations in crane hoist and railed electric vehicle applications, impulse-commutated SCR converters were mainly used with DC shunt motors in these applications. See, for example, U.S. Pat. Nos. 3,535,605; 3,551,771; 3,553,554; and 3,555,385.
Impulse-commutated SCR converters were relatively complicated low frequency devices, and bulky as a result of the requirement for commutation capacitors and/or reactors. Such converters were prone to failure under high current or fault conditions.
During the late 1970""s, reliable high power semiconductor switching devices, such as bipolar junction transistors (BJTs), became available. Such BJTs were employed in DC motor controllers during the 1980""s. For example, in 1985, Saftronics Inc., then located in Youngstown, N.Y., produced model 2BC-300 dual DC series motor choppers for electric vehicles, utilizing 400 A/600 V BJTs manufactured by Fuji. The motor""s field and armature were in series with the BJT, and a LEM 300 A Hall Effect current transducer was employed to obtain isolated current feedback. This controller made use of the well-known xe2x80x9ccurrent amplification effectxe2x80x9d to obtain high motor current during stall or low speed xe2x80x9cbreak awayxe2x80x9d conditions while drawing only a fraction of the motor current from a 320 VDC supply.
In the late 1980""s, an improved power switching device, the Insulated Gate Bipolar Transistor (IGBT), became available and was quickly used in many DC motor control applications, instead of BJTs. One such DC/DC controller was the IGBT-based model A 375 for DC series wound motors, as manufactured in 1989 by Saftronics Inc. of Fort Myers, Fla. This controller, rated for 320 VDC, employed an IGBT and current sensor connected in series with both the motor""s armature and field as configured in the 2BC-300 dual DC series motor choppers, in order to control the motive effort of a DC series traction motor. The model A 375 was applicable to both crane hoist and travel motion control, as well as motive control for railed and rubber-tired vehicles. However, it had the disadvantage that when applied to hoist control, it was difficult to maintain suitable light hook speed control. Also, braking control during lowering was very load-dependant.
During 1995, Saminco of Fort Myers, Fla. produced the IGBT-based model A812 DC/DC controller with separate control of the DC series wound motor""s field as well as armature, providing xe2x80x9cfield followerxe2x80x9d or series motor characteristics during motoring or hoisting, and shunt motor characteristics during regenerative braking conditions. The model A 812 is widely used for railed vehicle applications. However, it is not readily suitable for crane hoist applications without significant alterations to the method of connecting the controller to the industry-standard four-terminal hoist/brake assembly via sliding shoes on collector rails.
U.S. Pat. No. 5,875,281 discloses a microprocessor controlled hoist and travel motion controller, which employs a single IGBT and current transducer in series with the hoist motor""s armature and field during hoisting, as employed by the model A 375. However, unlike the model A 375, this controller provides separate field control during a xe2x80x9cLower Fastxe2x80x9d mode using a second IGBT to control the motor""s field. In both xe2x80x9cLower Slowxe2x80x9d and xe2x80x9cLower Fastxe2x80x9d modes, resistors are employed to dissipate energy generated during lowering. Although this controller employs the industry-standard four-terminal hoist assembly connections, it is only used with DC series wound motors and cannot readily provide independent field control during hoist xe2x80x9cRaisexe2x80x9d operations. It also requires a speed feedback device mounted on the hoist motor connected to the controller""s microprocessor in order to provide good speed control. Since the hoist motor is usually mounted on a moving structure, it would be very difficult and expensive to achieve this requirement.
For travel motions, the controller of U.S. Pat. No. 5,875,281 utilizes electro-mechanical switches in the series wound motor""s field to establish direction of motion of the crane. When it is desired to reverse motion when travelling in a given direction, the series motor""s field connections are reversed, and mechanical energy in the moving crane is dissipated in a resistor switched into the circuit by yet another electro-mechanical switch.
Many modern crane controllers for use with DC series motors in crane hoisting and travel applications are still of the xe2x80x9cconstant potentialxe2x80x9d contactor/resistor type, with one configuration used for hoisting, and a significantly different configuration used for travel (bridge and trolley) applications. These controllers use contactors which switch under load causing arcing during load break operations. This results in contactor tip burn out which requires frequent maintenance. In addition, much energy is wasted in the resistors during control operations. Furthermore, these controllers can severely stress motor life because of high voltage and current conditions that exist with this technology. Other disadvantages of such controllers include: (1) hook speed during hoisting is highly load dependent and can be relatively very high; (2) field current during low speed dynamic lowering can be as much as 250% of rated current causing possible premature motor damage due to overheating in severe duty applications; (3) armature voltage during high speed dynamic lowering can be as much as 200% of rated voltage giving rise to the possibility of DC motor commutator arc-over; (4) the resistors waste energy and create considerable heat; (5) the load-break contactor tips are a high maintenance item; (6) control can only be achieved in steps, since there are only a finite number of switched resistor stages; (7) it could be possible to overspeed the DC series motor during very light hook duty if the crane operator inadvertently applies full voltage to the hoist motor; (8) there could exist a delay between cessation of motor current at the end of a hoist RAISE motion and the setting of the series brake due to a time delay caused by the current in the series brake windings decaying slowly through a low impedance electrical pathxe2x80x94this could cause the load on the hook to sag; and (9) during hoisting, when the operator moves his master switch to xe2x80x9cOFFxe2x80x9d, deceleration of hook speed is determined only by upwards mechanical inertia against the force of gravity and this can vary according to load; therefore, there will be an uncontrolled coasting period during a hoisting operation.
DC shunt motors for crane duty applications are typically employed with SCR controllers powered from three-phase AC power sources.
FIG. 1 shows a typical electric overhead travelling crane 2 including a control panel 4, magnet control 6, manual magnetic disconnect 8, dynamic braking resistor rack 10, operator controllers 12, brakes 14, and power limit switch 16. The crane 2 further includes a hoist 18 for a bottom block 20 having a hook 21, a bridge 22, a trolley 24, a trolley motor 26, a bridge motor (direct wire) 28, a hoist motor 30, an end truck 32 and a runway 34. The arrows 36, 38 and 40 indicate the bridge, trolley, and load or hoist directions, respectively, of the crane 2.
FIG. 2 shows four quadrants (I, II, III, IV) of operation for a DC motor (M) 42 including a first quadrant 44 for positive speed (S) and positive torque (T) (e.g., power hoisting under relatively light or heavy load), a second quadrant 46 for positive speed and negative torque (e.g., braking hoisting motion under relatively light or heavy load), a third quadrant 48 for negative speed and negative torque (e.g., power lowering under relatively light load), and a fourth quadrant 50 for negative speed and positive torque (e.g., braking lowering motion under relatively heavy load).
There is room for improvement in motor controllers.
These and other needs are met by the present invention in which a universal microprocessor-based DC/DC controller provides a wide range of control applications for DC shunt, compound or series wound DC motors powered by a DC/DC converter.
As one aspect of the invention, a reversible direct current (DC) motor drive is for a DC motor having a speed and a torque, including first and second terminals for a field winding and third and fourth terminals for an armature winding, and operable in at least one of four quadrants including positive speed and positive torque, positive speed and negative torque, negative speed and negative torque, and negative speed and positive torque. The DC motor drive comprises: at least two input terminals adapted to receive a DC voltage, the input terminals including first and second input terminals; first and second switches electrically connected in series between the first and second input terminals; first and second transistors electrically connected in series between the first and second input terminals; first and second diodes electrically connected in parallel with the first and second transistors, respectively, the first and second transistors having a first polarity, the first and second diodes having an opposite second polarity; a third diode; a third transistor electrically connected in series with the third diode, the third transistor having the first polarity, the third diode having the opposite second polarity; at least three output terminals, the output terminals including a first output terminal electrically interconnected with a first node between the first and second switches, a second output terminal electrically interconnected with a second node between the first and second transistors, and a third output terminal electrically interconnected with a third node between the third diode and the third transistor; means for determining a voltage between the first and second output terminals; means for determining a first current flowing between the second node and the second output terminal, and a second current flowing between the third node and the third output terminal; means for calculating the speed of the DC motor from the determined voltage, the determined first current and the determined second current; and means for controlling the first, second and third transistors and responding to the calculated speed of the DC motor to provide operation of the DC motor in the four quadrants and independent control of the first and second currents.
Preferably, the first and second input terminals have a capacitor electrically connected therebetween. The means for controlling includes a third switch and a fourth switch, with the third switch electrically connected in series with a resistor, the series combination of the third switch and the resistor being electrically interconnected between a third input terminal and the first input terminal, the fourth switch electrically interconnected between the first and third input terminals. The means for controlling further includes means for closing the third switch, means for sensing a voltage of the capacitor, and means for closing the fourth switch after the voltage of the capacitor is above a predetermined value.
The first and second terminals of the DC motor may be electrically interconnected in series with the third and fourth terminals of the DC motor. The first output terminal is adapted for electrical interconnection with the first terminal of the DC motor, the second output terminal is adapted for electrical interconnection with the second terminal of the DC motor, and the third output terminal is adapted for electrical interconnection with the fourth terminal of the DC motor.
The first and second output terminals may be adapted for electrical interconnection with the first and second terminals, respectively, of the DC motor, and the third and fourth output terminals may be adapted for electrical interconnection with the third and fourth terminals, respectively, of the DC motor.
A fourth output terminal may be electrically interconnected with the first input terminal.
As another aspect of the invention, a reversible direct current (DC) motor drive is for a DC motor having a speed, including first and second terminals for a field winding having a field voltage and a field current, and third and fourth terminals for an armature winding having an armature voltage and an armature current. The DC motor drive comprises: a DC/DC converter having a DC input and at least three output terminals, the output terminals including a first output terminal having a first pulsed DC voltage with a first DC voltage value, a second output terminal having a second pulsed DC voltage with a second DC voltage value, and a third output terminal having a third pulsed DC voltage with a third DC voltage value, the first and second output terminals adapted for electrical interconnection with the first and second terminals of the DC motor, the third output terminal adapted for electrical interconnection with the fourth terminal of the DC motor; means for determining the armature voltage of the armature winding of the DC motor; means for determining the field current of the field winding of the DC motor; means for determining the armature current of the armature winding of the DC motor; means for calculating the speed of the DC motor from the determined armature voltage, the determined field current and the determined armature current; means for providing a speed reference; and means for controlling the DC/DC converter responsive to the calculated speed of the DC motor and the speed reference in order to independently control the first, second and third DC voltage values.
Preferably, the means for controlling includes a nested loop structure including an outer control loop for the speed of the DC motor and two separately controllable inner control loops for the armature current and the field current of the DC motor.
As a further aspect of the invention, a reversible direct current (DC) motor drive is for a DC motor having a speed and a torque, including first and second terminals for a field winding and third and fourth terminals for an armature winding, and operable in at least one of four quadrants including positive speed and positive torque, positive speed and negative torque, negative speed and negative torque, and negative speed and positive torque. The DC motor drive comprises: at least two input terminals adapted to receive a DC voltage, the input terminals including first and second input terminals; first and second switches electrically connected in series between the first and second input terminals; first and second transistors electrically connected in series between the first and second input terminals; first and second diodes electrically connected in parallel with the first and second transistors, respectively, the first and second transistors having a first polarity, the first and second diodes having an opposite second polarity; a third diode; a third transistor electrically connected in series with the third diode, the third transistor having the first polarity, the third diode having the opposite second polarity; at least three output terminals, the output terminals including a first output terminal electrically interconnected with a first node between the first and second switches, a second output terminal electrically interconnected with a second node between the first and second transistors, and a third output terminal electrically interconnected with a third node between the third diode and the third transistor; means for determining a voltage between the first and second output terminals; means for determining a first current and a second current, the first current flowing between one of the first node and the first output terminal, the second node and the second output terminal, and the third node and the third output terminal, and the second current flowing between a different one of the first node and the first output terminal, the second node and the second output terminal, and the third node and the third output terminal; means for calculating the speed of the DC motor from the determined voltage, the determined first current and the determined second current; and means for controlling the first, second and third transistors and responding to the calculated speed of the DC motor to provide operation of the DC motor in the four quadrants and independent control of the first and second currents.